Washington: President Barack Obama and Republican challenger Mitt Romney began their pivotal third and last encounter focused entirely on foreign policy with a fiery clash over the controversy surrounding the Sep 11 attack on the US mission in Libya.

Winning third coin toss in a row Romney got the first question from moderator Bob Schieffer of CBS News during the Monday night debate divided into six 15-minute segments focusing on six topics at Lynn University in Boca Raton, Florida.

Suggesting that the hopes raised by the Arab spring had been belied and radical violent extremism had taken hold in 10 or 12 countries in the Middle East presenting an "enormous threat to our friends, to the world, to America long term."While congratulating Obama for taking out Osama bin Laden and going after the leadership in Al Qaeda, Romney said the US "can't kill our way out of this mess" and Washington must have a comprehensive strategy to help reject this kind of extremism.

Obama, in turn, said that he had focused on keeping the American people safe over the last four years, had ended the war in Iraq, refocused attention on those who actually killed on 9/11 and as a consequence, Al Qaeda's core leadership has been decimated.

On Libya, vowing to go after those who killed Americans and bring them to justice, he said Americans took leadership in organising an international coalition that was able to liberate a country that had been under the yoke of dictatorship for 40 years and got rid of a despot who had killed Americans.

Unlike last week's contentious town hall-style debate in which the candidates ambled around the stage and sparred with each other as they answered questions from the audience made up of 82 undecided voters, Obama and Romney were seated at a table with Schieffer.

India is unlikely to figure in the debate as it's not a contentious issue with both Democrats and Republican claiming credit for transforming India-US ties into a strategic partnership over three administrations starting with former President Bill Clinton.

But it may get a passing mention as the contenders debate America's role in the world, Our longest war Afghanistan and Pakistan, Red Lines Israel and Iran, The Changing Middle East and the New Face of Terrorism I and The Changing Middle East and the New Face of Terrorism II and The Rise of China and Tomorrow's World.

Monday's debate is crucial for both Obama and Romney as the latest polls continue to show the White House a dead heat just two weeks before the Nov 6 election with Obama maintaining very narrow advantages in crucial battleground states like Ohio.

The latest poll confirming the trend is a new Washington Post-ABC News poll showing the race still deadlocked among likely voters nationally with 49 percent backing Obama and 48 percent favouring Romney.